Karl Landsteiner, the guy who classified blood types

More than a hundred years ago, a very intelligent man named Karl Landsteiner discovered that we, humans, have different blood types and categorized them into: A, B, AB and O. Knowing the blood type allows us to make safe transfusions from human to human without harming the one receiving the new blood.

In 1901 Karl Landsteiner discovered these blood types and, since then, we now know to a whole lot with the blood and there are over 107 million blood donations every year.